Detection of electronegative compositions by means of electron capture detection



Oct. 4, 1966 K. P. DIMICK ET AL 3,277,296 DETECTION OF ELEGTRONEGATIVE COMPOSITIONS BY MEANS OF ELECTRON CAPTURE DETECTION Filed March 1, 1963 ELECTROMETER ADUSTABLE POWER SUPPLY RECORDER H. HARTMANN ATTORNEYS United States Patent ()fiice Patented Oct. 4, 1966 3,277,296 DETECTION OF ELECTRONEGATIVE COMPOSI- TIONS BY MEANS OF ELECTRON CAPTURE DE- TECTION Keene P. Dimick, Santa Rosa, and Charles H. Hartmann,

Moraga, Calif., assignors to Wilkens Instrument & Research, Inc., Walnut Creek, Calif.

Filed Mar. 1, 1963, Ser. No. 262,020 7 Claims. (Cl. 250-435) This invention relates to an improved method and means for the detection of small amounts of solute in a carrier gas solvent. More specifically, it relates to the quantitative detection of an electronegative solute in a non-electronegative carrier gas through the capture of free electrons in an electric field wherein the relationships which create the free electrons and electric field have been modified to provide advantages over prior techni-ques.

While the present device has utility in any of those areas where it is desired to detect the presence of lectronegative compositions, one of the areas contemplated [for the use of the present invention is in combination with a chromatograph. In this case the present invention may advantageously be used to analyze and detect quantitatively the components in the eflluent from the chrornatograph column. The invention will be described with particular emphasis on such a combination, where appropriate, for clarity and simplicity of description.

In the accompanying drawings there is shown in:

FIG. 1 in side elevation and partially in section an electron capture detector embodying the present invention.

FIG. 2 is a sectional view taken along the line 22 of FIG. 1.

A number of different types of detectors have been developed in recent years for analyzing the compositions in a gas carrier. A number of these devices are authoritatively described and discussed in Lovelock, J. E., Anal. Chem, 33, 162-l78 (1961). The present invention provides an improvement upon one of the types described in the above article; namely, the electron capture detector.

Broadly, the present invention in common with other electron capture detector methods and apparatus is used for detecting the presence of an electronegative composition as solute in a non-electronegative gaseous carrier. As used herein the term electronegative refers to molecular compositions of matter in which the molecules exhibit electron afiinity or an ability to pick up free electrons and form negative ions analogous to similar tendencies of certain well known atomic species, e.-g. oxygen and chlorine. The present invention may be used for the detection of all such materials alone or in combination, and particularly for molecules such as alkyl halides, conjugated carbonyls, nitriles, nitrates, and organometals. The present invention is especially suitable for the detection of halogenated pesticides. The term non-electronegative is used herein with reference to the carrier gas refers to molecules which do not exhibit electron afiinity and do not form negative ions. Examples of such materials and which are suitable for use in the present invention include nitrogen, hydrogen, helium, argon, alkanes in general, and combinations of gases such as methane with argon.

Electron capture detectors in general respond to the presence of a suitable solute in a suitable carrier gas by reference to the loss of signal rat-her than with reference to a positively produced electrical current as is true in other types of detectors. For example, if a nitrogen carrier gas is flowed through such a detector and a suitable radioactive source is employed to ionize the nitrogen molecules, slow electrons are formed. These slow electrons migrate to the anode in the detector cell under a fixed voltage which is termed cell voltage. Collected, these slow electrons produce a steady current which may be measured by an electrometer suitably linked with the detector cell. If a sample such as a halogenated pesticide is introduced into the nitrogen carrier gas, the electron absorbing pesticide molecules absorb or capture some of these free electrons and thereby reduce the current. The loss of current is a measure of the amount and electron affinity of the pesticide compound. Through the use of techniques known in the art such as the construction of calibration curves from the observed or recorded currents, the desired quantitative information on the detected solute can be obtained.

The present invention operates in accordance with the above principles and in one aspect there is provided a system for the detection of electronegative compositions comprising means establishing a flow of a non-electronegative carrier gas containing an electronegative solute to be detected, means defining a generally cylindrical substantially fluid-tight detection zone having an input opening adjacent one end and an outlet opening spaced therefrom, a cylindrical source of ionizing radiation emitting at least about 200 millicuries of radioactive particles disposed concentrically interiorly of said detection zone defining means, conduit means for passing said flow of carrier gas and solute into said inlet opening for passage through said detection zone substantially without atmospheric contamination and then through said outlet opening, means creating an electrical field interiorly of said detection zone including a first cylindrical electrode within said zone, and a second oppositely charged electrode Within said zone axially aligned with said first electrode and spaced outwardly therefrom whereby radiation from said source ionizes said carrier gas passing through the detection zone and free electrons thereby produced or collected on one of said electrodes to create an electron current, and means for measuring the decrease in said ionized gas electron current that is caused by the capture of free electrons by the electronegative solute in said carrier gas.

The foregoing system and particularly the construction of the detection zone offers a number of advantages. The cylindrical construction enables the cell to be made to have a small volume in an exceptionally leak-proof manner. The leak-free aspect of the present detector zone or cell eliminates the problems of prior devices caused by the difiusion of oxygen from the ambient atmosphere. Since the present unit is not plagued by back diffusion, the purge gas conventionally found in prior methods and apparatus may be dispensed with. The cylindrical configuration enables the detection zone to be of a relatively smaller volume then prior devices, thereby enhancing the detectors sensitivity and promoting quicker and truer response to the solute in the carrier gas. Closely related to this advantage is the fact that the geometry of the present unit permits relatively low flow rates While still achieving the desired sensitivity.

The relationship of the present electrodes provides other advantageous properties. The axially aligned electrode spaced outwardly from the cylindrical electrode creates a nonuniform electrical field which has been demonstrated to provide the necessary field strengths for achieving optimum or near optimum sensitivities over a relatively wide range of voltages with various substances analyzed. This is in distinction with the critical nature of the narrow voltage range which may be employed in order to obtain optimum sensitivity in prior electron capture detectors for a particular solute. The sum of these features and advantages is to provide an outstandingly sensitive detector. The present detector has other advantages (such as case of cleaning the source) which will become apparent as the detailed description of the drawings proceeds.

Other terms used herein in discussing the present invention are defined as follows.

Standing current: The measured current produced in the detector cell from collected electrons which are formed by the ionization of carrier gas molecules Cell potential: The voltage applied across the two detector cell electrodes (anode and cathode).

Source: The substance emitting the radioactive particles for ionizing the carrier gas molecules.

Dynamic range: The relationship between sample concentration and detector response. The linear portion of the dynamic range is referred to as the linearity of the detector.

Turning to the drawings, the present improved electron capture detector may include a suitable base upon which the detection zone can be supported. Base 10 may actually be part of a chromatograph where the detector is used in combination therewith. The detection zone itself is encased within a protective shell 11.

The detection cell shown generally at 13 comprises a generally cylindrical body 14 joined through a suitable electrically insulating seal 15 to an axially aligned tube 16. Opposite from the supported end of body 14, an inert resinous plug 17 is sealably inserted. Plug 17 may be made from a suitable inert material such as polytetratfiuoroethylene resin (commercially available under the trademark Teflon). Plug 17 defines a canal 18 therethrough for the discharge of carrier gas and solute from within body 14.

concentrically disposed within body 14 is a radioactive source 19. Source '19 may be suitably mounted on a plurality of resilient spring-like wires 20. In the preferred embodiment source .19 comprises a stainless steel foil in cylindrical form composed of titanium metal plated on one side (facing inwardly) and tritium occulded therein. The tritium source produces beta particles having a total activity of at least about 200 and preferably about 250 millicuries. A source having at least about 200 millicuries total activity is essential to the proper functioning of the present detector, with an activity in the range of about 200-300 millicuries being most suitable. Activities of less than about 200 millicuries fail to provide the requisite sensivity in the present detector.

A suitable carrier gas and solute to be detected (as from a chnomatograph column) is passed into body 14 from tube '16. The gas enters tube 16 from its origin through tube 21 which is in fluid communication with tube 16. Tubes 16 and 2 1 are suitably joined together in fluid-tight relation by means of a joint comprising an inert resinous sealing washer 22, a hex nut 23, and a coop. eratively threaded tubular neck 24 through which the gas from tube -21 is passed to tube 116. As noted the carrier gas passing through body 14 is exhausted through canal 18 in plug 17. The diameter of canal 18 is small enough to prevent back diffusion of atmospheric oxygen under normal flow rates.

While within body 14 the carrier gas and solute is subjected to radioactive bombardment from the source 19 whereby molecules of the non-electronegative carrier gas are ionized to produce free electrons. These free electrons are subjected to an electrical field created between an anode and a cathode. In the preferred embodiment the anode function is performed by tube 16 by forming tube 16 from metal. The metal is preferably an alloy having a coefiicient of expansion similar ot that of hard glass. An example of such a material is an alloy consisting of 20% nickel, 17% cobalt, 12% manganese, and the balance iron. Such a material is commercially available and is known by the trademark Kovar. Anode tube 16 is electrically linked through a suitable lead 24 to a terminal 25 in shell 11 and then to additional components to be mentioned hereinafter.

In the preferred embodiment the cathode function is performed by the source 19' which includes a stainless steel foil as noted. Source 19 is suitably linked by means of an electrical lead 26 through a terminal 27 to the re mainder of the circuit. The remainder of the circuit to which terminals 25 and 2 7 are connected may comprise any suitable conventional circuit that is used in combination with the various types of detectors known in the art. Brierfiy, the circuit may comprise an adjustable power supply 23, an electrometer 29, and a recorder #30. Those skilled in the art will appreciate the many variations and modifications that are suitably incorporated in such a circuit.

Body 14 is formed from any suitable material. Thus body 14 may comprise .glass or, since it is separated from tube 16 by insulation 15, may also and preferably be formed from a Kovar type material.

In operation, the power supply 28 is adjusted so that a suitable potential is created across the electrodes within body 14. Because of the configuration of the electrodes, i.e. cylindrical source 19 and tube 16 spaced axially and outwardly therefrom, a nonuniform field is created and the voltage adjustment is relatively not critical in obtaining the desired sensitivity. A fixed potential of volts has been found to be most suitable when the present invention is used for routine pesticidal analysis.

The bombardment of the carrier gas such as nitrogen by the tritium source creates free electrons which are collected by the anode (tube 16) and are measured most conveniently in terms of microvolts. The current produced is known as the standing current of the cell. Generally, a high standing current as from a clean or new cell will yield higher sensitivity, wider dynamic range, and triuer detector response than a cell with a low standing current. One way of measuring the standing current is to observe the difference in recorder deflection with the cell 13 connected and disconnected from the electrometer 29. With a clean new source a value about 4,000-5,000 microvolts may be employed for many purposes with good results.

The presence of a solute to be detected in the carrier gas will capture the electrons formed by ionization of the carrier gas and thereby reduce the standing current and is observable on the recorder.

The 4,0005,000 microvolt level of the standing current generally tends to drop with use for a number of reasons at which time a clean up of the source is advisable. Clean up is easily accomplished by removing plug 17 and lifting source 19 from body 14. Cleaning of the source is beneficially accomplished by immersing the source in 5% alcoholic potassium hydroxide in combination with vibration for 15 minutes in an ultrasonic cleaner or the like.

The sensitivity of the present invention is most surprising and outstanding. As an illustration, the pesticide Lindane in a nitrogen carrier gas was passed through the above described detector cell. The smallest sample measured was 0.03 nanogram, while the smallest size which could be detected was .002 nanogram. (A nanogram is .001 microgram. 1 nanogram in 1 microliter of solution equals 1 p.p.m.) In constructing a calibration curve for Lindane with a nitrogen carrier gas, a dynamic range over the linear portion of the curve of about 1,000 was obtained.

Although the foregoing invention has been described in some detail by way of illustration and example for purposes of clarity of understanding, it is understood that certain changes and modifications may be practiced within the spirit of the invention as limited only by the scope of the appended claims.

What is claimed is:

1. A system for the detection of electronegative compositions comprising means establishing a flow of a nonelectronegative carrier gas containing an electronegative solute to be detected, means defining a generally cylindrical substantially fluid-tight detection zone having an input opening adjacent one end and an outlet opening spaced therefrom, a cylindrical source of ionizing radiation emitting at least about 200 millicuries of radioactive particles disposed interiorly of said detection zone defining means, conduit means for passing said flow of carrier gas and solute into said inlet opening and through said detection zone substantially without atmospheric contamination and then through said outlet opening, means creating an electrical field interiorly of said detection zone and including a first cylindrical electrode in said zone and a second oppositely charged electrode in said zone axially aligned with said first electrode and spaced entirely outwardly therefrom, whereby radiation from said source ionizes the carrier gas passing through the detection zone and free electrons thereby produced are collected on one of said electrodes to create an electron current, and means for measuring the decrease in said ionized carrier gas electron current that is caused by the capture of free electrons by the electro-negative solute in said carrier gas.

2. A system in accordance with claim 1 wherein said first cylindrical electrode is a cathode and is concentrically disposed interiorly of said detection zone and wherein said first cylindrical electrode also contains said radioactive source.

3. A system in accordance with claim 1 wherein said source of radiation comprises a tritium foil emitting about 200-300 millicuries of beta particles.

4. A system in accordance with claim 1 wherein said electro-negative solute is a halogenated pesticide and said non-electronegative carrier gas comprises nitrogen.

5. A system in accordance with claim 1 wherein said second electrode is an anode and comprises a tube axially joined to one end of said means defining said cylindrical detection zone, said tube being made from a metallic composition having a coeflicient of expansion approximately that of hard glass, said tube thereby serving as both an electrode and a conduit for passing carrier gas flow into said detection zone.

6. A system in accordance with claim 1 wherein the potential between said electrodes is on the order of volts.

7. A system for the detection of electronegative compositions comprising means establishing a flow of nonelectronegative carrier gas containing an electronegative solute to be detected, a generally cylindrical tube defining a detection zone interiorly thereof, an inlet tube axially aligned and joined in insulated relationship at one end of said cylindrical tube and lying entirely outwardly there from, said inlet tube being made from a metallic composition having a coefiicient of expansion approximately that of hard glass for conducting said flow of carrier gas and solute into said cylindrical tube, an inert resinous plug removably and sealably disposed in the end of said cylindrical tube opposite from said inlet tube and having a discharge port therein for the discharge of carrier gas and solute from said cylindrical tube, a cylindrical metallic foil containing tritium emitting at least about 200 millicuries of radioactive particles, said foil being disposed concentrically interiorly of said cylindrical tube and spaced longitudinally apart from said inlet tube, means establishing a preselected potential between said foil and inlet tube, and means associated with said potential establishing means for measuring changes in the potential caused by the passage of said carrier gas and solute through said cylindrical tube.

References Cited by the Examiner UNITED STATES PATENTS 2,943,196 6/1960 Eickoif 250--44 X 3,104,320 9/1963 Speakman et al. 250-435 3,176,135 3/1965 Lovelock 250--43.5 X

RALPH G. NILSON, Primary Examiner.

ARCHIE R. BORCHELT, Examiner. 

1. A SYSTEM FOR THE DETECTION OF ELECTRONEGATIVE COMPOSITIONS COMPRISING MEANS ESTABLISHING A FLOW OF A NONELECTRONEGATIVE CARRIER GAS CONTAINING AN ELECTRONEGATIVE SOLUTE TO BE DETECTED, MEANS DEFINING A GENERALLY CYLINDRICAL SUBSTANTIALLY FLUID-TIGHT DETECTION ZONE HAVING AN INPUT OPENING ADJACENT ONE END AND AN OUTLET OPENING SPACED THEREFROM, A CYLINDRICAL SOURCE OF IONIZING RADIATION EMITTING AT LEAST ABOUT 200 MILLICURES OF RADIOACTIVE PARTICLES DISPOSED INTERIORLY OF SAID DETECTION ZONE DEFINING MEANS, CONDUIT MEANS FOR PASSING SAID FLOW OF CARRIER GAS AND SOLUTE INTO SAID INLET OPENING AND THROUGH SAID DETECTION ZONE SUBSTANTIALLY WITHOUT ATMOSPHERIC CONTAMINATION AND THEN THROUGH SAID OUTLET OPENING, MEANS CREATING AN ELECTRICAL FIELD INTERIORLY OF SAID DETECTION ZONE AND INCLUDING A FIRST CYLINDRICAL ELECTRODE IN SAID ZONE AND A SECOND OPPOSITELY CHARGED ELECTRODE IN SAID ZONE AXIALLY ALIGNED WITH SAID FIRST ELECTRODE AND SPACED ENTIRELY OUTWARDLY THEREFROM, WHEREBY RADIATION FROM SAID SOURCE IONIZES THE CARRIER GAS PASSING THROUGH THE DETECTION ZONE AND FREE ELECTRONS THEREBY PRODUCED ARE COLLECTED ON ONE OF SAID ELECTRODES TO CAUSE AN ELECTRON CURRENT, AND MEANS FOR MEASURING THE DECREASE IN SAID IONIZED CARRIER GAS ELECTRON CURRENT THAT IS CAUSED BY THE CAPTURE OF FREE ELECTRONS BY THE ELECTRO-NAGATIVE SOLUTE IN SAID CARRIER GAS. 